The aims of our experimental marrow stem cell transplantation (SCT) program are firstly to improve the outcome after allogeneic SCT by optimizing the stem cell and lymphocyte doses of the transplant, and secondly to exploit the antitumor effect of donor immune cells to treat hematological and non-hematological malignant diseases by adoptive immunotherapy. Analysis of the last 50 patients receiving blood stem cell transplants indicate that our strategy of T cell depletion and delayed lymphocyte add-back confers a low risk of severe graft-versus-host disease (GVHD) and transplant-related mortality. Results also confirm the beneficial effect of high stem cell doses on disease-free survival found in earlier trials. We are now developing a technique to selectively deplete SCT of alloreacting lymphocytes which cause GVHD. In a new experimental transplant protocol we will test whether this technique prevents GVHD while permitting useful immune recovery. To study the antitumor effect of donor immune cells we have treated over 80 patients using non-myeloablative (low intensity) SCT. Three categories of individuals have been treated in this way: 1) patients over 55 years, normally considered too old to withstand a conventional bone marrow transplant; 2) younger but debilitated patients who are considered at high risk of complications from a standard transplant; 3) transplants to confer a graft versus-tumor effect in metastatic renal cell carcinoma, metastatic melanoma and other metastatic cancers not amenable to standard treatments. Preliminary results indicate that the transplant procedure is well-tolerated, but that GVHD is the major cause of death in patients over 50 years. We therefore plan to use the selective T cell depletion approach to prevent GVHD in these non-myeloablative transplants. The complete and stable resolution of metastases in four patients with renal cell carcinoma and the absence of any detectable minimal residual disease in four patients with chronic myeloid leukemia is proof of principle that the transplant approach can exert a graft-versus-tumor effect. Our laboratory research has four objectives: 1) the detection and expansion of graft versus-leukemia/tumor lymphocytes to study interactions between alloreacting T cells and malignant cells; 2) the discovery of new leukemia antigens capable of initiating a graft-versus-leukemia (GVL) response in allogeneic T cells; 3) development of techniques to select and expand leukemia- and tumor-reacting T cells for adoptive immunotherapy of patients with malignant diseases; and 4) development of assays to detect antigen-specific T cells for the direct monitoring in vivo of specific T cell responses to leukemia, tumor and viral antigens. To further study interactions between T cells and normal or abnormal bone marrow cells, we are evaluating the use of immunosuppressive therapy to improve hematological function in myelodysplastic syndrome (MDS) - a disease believed to involve immune-mediated marrow suppression. Patients with MDS are treated with antithymocyte globulin (ATG) and cyclosporine as immunosuppression and monitored for recovery of bone-marrow function. Based on previous results we anticipate a response of over 30% in these patients. Laboratory studies show that ATG treatment prevents a direct inhibitory effect of T cells on marrow progenitors and suggest that marrow failure in MDS is due to an autoimmune process.